1. Field of the Invention
The present invention relates to surface acoustic wave devices used for, for example, resonators, and bandpass filters, and more particularly, to a surface acoustic wave device including an insulator layer that is arranged so as to cover an IDT electrode and the manufacturing method therefor.
2. Description of the Related Art
Duplexers (DPX) and RF filters used in mobile communication systems must have broadband characteristics and good temperature characteristics. In a surface acoustic wave device conventionally used for DPXs or RF filters, a piezoelectric substrate made of 36° to 50° rotation-Y plate X propagation LiTaO3 is used. The frequency temperature coefficient of the piezoelectric substrate is in the range of −45 ppm/° C. to −35 ppm/° C. In order to improve temperature characteristics, a method of forming a SiO2 film having a positive frequency temperature coefficient so as to cover an IDT electrode on the piezoelectric substrate is known.
However, in a structure in which a SiO2 film is formed so as to cover an IDT electrode, a step is produced between a portion in which the finger electrode of the IDT electrode is disposed and a portion in which the IDT electrode is not disposed. In other words, a difference in the height of the surface of the SiO2 film between a portion in which the IDT electrode is disposed and a portion in which no IDT electrode is disposed, has to be present. Consequently, there has been a problem in that insertion loss degrades due to unevenness of the surface of the above-mentioned SiO2 film.
Moreover, as the film thickness of the IDT electrode increases, the above-mentioned unevenness also increases. Accordingly, the film thickness of the IDT electrode cannot be substantially increased.
As a method for solving such problems, in Japanese Patent Laid Open Publication No. 2004-112748 (Patent Document 1), a method is disclosed in which a SiO2 film is formed so as to cover the IDT electrode and a first insulator layer after the first insulator layer, which has a film thickness approximately equal to that of the electrode, is formed between electrode-fingers of the IDT electrode. Here, since the substrate is substantially flat, the surface of the SiO2 film is flattened. In a surface acoustic wave device described in Patent Document 1, the above-mentioned IDT electrode includes a lamination film composed of a metal having density greater than that of Al, or an alloy primarily composed of the metal, and another metal, and the density of the electrode is at least about 1.5 times that of the first insulator layer.
On the other hand, in Japanese Patent Laid Open Publication No. 09-83030 (Patent Document 2), a surface acoustic wave device is disclosed, a groove is formed in one side of a LiTaO3 substrate or a LiNbO3 substrate, and an IDT electrode is formed by filling the groove with Al.
However, in the surface acoustic wave device described in Patent Document 1, since an electrode having a density greater than Al is used, the fluctuations of acoustic velocities and frequencies due to the fluctuations in the electrode thickness are relatively large, while it has been shown that, when an electrode formed of Al is provided, the reflection coefficient of the electrode is relatively low. As a result, sufficient characteristics for a surface acoustic wave resonator or a surface acoustic wave filter cannot be obtained.
In the surface acoustic wave device described in Patent Document 2, the electrode is formed by filling Al in the groove formed in the LiTaO3 substrate or the LiNbO3 substrate. Therefore, when a material layer is laminated on the substrate so as to cover the electrode, the surface of the material layer is flattened. However, when a surface acoustic wave filter is configured with a surface acoustic wave resonator, sufficient characteristics cannot be obtained.